I used to have a similar device. They use infrared light, so usually they only measure the surface temperature of the water, which could be quite a bit cooler than what's underneath. Infrared light does not penetrate water very well.

Yeah, I thought it was most likely infrared. And I too wondered about the surface temp giving a false reading.

When using an attached probe style Digital Thermometer, I find that raising the Yuzamashi or whatever I am using and giving it a few gentle swirls mixes the water sufficiently to give a reading more indicative of water below the surface even when reading near the surface.

I've got one of these on the way to me this week. I mainly want it for checking to make sure my carafe water is still hot or needs to be topped off, as well as the surface temp of the brewing pot. I figure an IR thermometer and a shake is more convenient than sticking a probe in the water every time.

If I measure with a digital thermometer inserted into the water and scan with this thing on the surface, the difference shows less than half a degree. It's accurate enough, and certainly more convenient.

Kevangogh wrote:If I measure with a digital thermometer inserted into the water and scan with this thing on the surface, the difference shows less than half a degree. It's accurate enough, and certainly more convenient.

Cool!

Anyone know if the IR type measures the surface temp of clear water or does it read further down?

Kevangogh wrote:If I measure with a digital thermometer inserted into the water and scan with this thing on the surface, the difference shows less than half a degree. It's accurate enough, and certainly more convenient.

Cool!

Anyone know if the IR type measures the surface temp of clear water or does it read further down?

Infrared radiation (IR) doesn't penetrate water too well. This is why water looks blue, because red light gets absorbed by water which then passes a larger proportion of blue light to our eyes. IR is just past red in the electromagnetic spectrum, so it is effectively even "redder" than what our eyes can see.

This effect is exploited by certain measurement devices in the manufacturing industry. You can measure the thickness of a water-based coating by seeing how much IR it absorbs.

EDIT: To answer the question, no, it pretty much just measures the surface. Sorry for getting off on a work-related lecture.